yii\validators\FileValidator::validateAttribute PHP Method

    public function validateAttribute($model, $attribute)
    {
        if ($this->maxFiles != 1) {
            $files = $model->{$attribute};
            if (!is_array($files)) {
                $this->addError($model, $attribute, $this->uploadRequired);
                return;
            }
            foreach ($files as $i => $file) {
                if (!$file instanceof UploadedFile || $file->error == UPLOAD_ERR_NO_FILE) {
                    unset($files[$i]);
                }
            }
            $model->{$attribute} = array_values($files);
            if (empty($files)) {
                $this->addError($model, $attribute, $this->uploadRequired);
            }
            if ($this->maxFiles && count($files) > $this->maxFiles) {
                $this->addError($model, $attribute, $this->tooMany, ['limit' => $this->maxFiles]);
            } else {
                foreach ($files as $file) {
                    $result = $this->validateValue($file);
                    if (!empty($result)) {
                        $this->addError($model, $attribute, $result[0], $result[1]);
                    }
                }
            }
        } else {
            $result = $this->validateValue($model->{$attribute});
            if (!empty($result)) {
                $this->addError($model, $attribute, $result[0], $result[1]);
            }
        }
    }

 public function testValidateAttributeErrNoTmpDir()
 {
     $m = $this->createModelForAttributeTest();
     $val = new FileValidator();
     $val->validateAttribute($m, 'attr_err_tmp');
     $this->assertTrue($m->hasErrors('attr_err_tmp'));
     $this->assertSame(Yii::t('yii', 'File upload failed.'), current($m->getErrors('attr_err_tmp')));
 }